Out of this World

In 2012 experimentalists, after decades of waiting, finally caught sight of the Higgs. The Large Hadron Collider did the job it was built to do.

The LHC is now hunting for physics beyond the standard model — and theoreticians have provided a host of speculative concepts to consider. Might the LHC see extra dimensions? Might it create mini black holes? Or (whisper it) might it find nothing beyond the standard model?

The LHCb experiment at CERN has today announced the discovery of a new baryon: the quark content of the Ξcc++ is ccu — in other words it contains two charm quarks and one up quark.

The commonest baryons contain combinations of u (up) and d (down) quarks. The proton and the neutron, for example, are uud and udd respectively. Baryons containing s (strange) quarks have long been known: the Ω–, which was discovered in 1964, has a quark content of sss. But up until now the heavier quarks (charm (c), bottom (b), top (t)) have only ever appeared singly in baryons. The Ξcc++ baryon contains two heavy quarks.

It’s important to note that the LHCb experiment hasn’t discovered a new fundamental particle. The Ξcc++ baryon is a permissible collection of bound quarks. But it is the first time that anyone has seen a baryon containing two heavy quarks. The Ξcc++ should allow physicists to explore the theories behind the Standard Model in ever more detail.

On 8 October 2013, the Nobel prize for physics was awarded to Francois Englert and Peter Higgs. In one sense this was a long time coming: the theoretical work that won the prize took place in 1964 (Englert, and his late colleague Robert Brout, working independently of Higgs, published first; a few weeks later Higgs published a paper that explicitly predicted the existence of a scalar boson; another group of physicists – Gerald Guralnik, Carl Hagen and Tom Kibble – published related work later in the same year). In another sense the prize was awarded remarkably quickly: experimental proof of the existence of a fundamental boson was announced on 4 July 2012, and it wasn’t until 14 March 2013 that it was confirmed to be a scalar (spin-0) boson. (If you want to learn more about the Higgs mechanism, you can find a variety of explanations here.)

To my mind, the discovery of the Higgs is one the crowning achievements of human civilisation: it is the culmination of a process that began 2500 years ago with the Greeks. Physicists now have a standard model of fundamental particles: there exists a small number of spin-1/2 point particles (6 quarks; the electron, muon and tau each with their associated neutrino) which interact via the exchange of spin-1 particles that mediate the electroweak and strong (these exchange particles being the photon; W+, W– and Z0; 8 gluons). In the ‘pure’ theories underpinning this model the fundamental particles are massless; they acquire mass – and thus in a certain sense their very existence – by interacting with a spin-0 field that pervades the entire universe. This spin-0 field has an associated particle; the Higgs boson. And that’s it. End of story. Except…

We are really just at the beginning of the story. The theories underpinning the standard model are in conflict with the other central pillar of physics: general relativity. The standard model is based on quantum physics; general relativity is a classical theory. Physicists need to develop a quantum theory of gravity. Furthermore, we now know that the standard model applies to only 5% of the universe: 95% of the mass-energy content of the universe resides in the so-called ‘dark’ sector. We desperately need to understand the nature of dark matter and dark energy.

Now that the Large Hadron Collider has discovered the Higgs its next job, when it becomes operational again after its current upgrade, is to shed light on the dark sector.

In a recent article Robin McKie presents his list of the 10 best physicists. Such lists are essentially meaningless – but it’s fun to argue and think about them! So here’s my own list of the 10 greatest physicists.

Isaac Newton – couldn’t not be on the list. Calculus, gravitation, laws of motion, optics, reflecting telescope and so on and so on and so on. I’m not ordering this list, but if I were Newton would be top.

Albert Einstein – special relativity, mass-energy equivalence, photoelectric effect, Brownian motion – all in one year! And general relativity on top of that. If I were ordering this list, which I’m not, Einstein would be second.

James Clerk Maxwell – electromagnetism, one of the great unifications in physics. Shame he’s not more widely known.

Galileo Galilei – Einstein called him the father of modern science. Galileo has to be on the list.

Archimedes – he doesn’t often appear on these “top-10” lists, but Archimedes was the leading scientist of antiquity.

Michael Faraday – discoveries include electrolysis, diamagnetism, electromagnetic induction; his experimental work formed the basis for Maxwell’s theories. One of the greatest experimentalists of all time.

Ernest Rutherford – the greatest experimentalist since Faraday. The father of nuclear physics.

Paul Dirac – early fundamental work in quantum mechanics and quantum electrodynamics; predicted the existence of antimatter; the Dirac equation describes the behaviour of fermions.

Enrico Fermi – one of my favourite physicists. He was the last of the great physicists who excelled in both experiment and theory.

John Bardeen – the only person to have one the Nobel prize for physics on two occasions. His work has had a huge impact on all our lives.

Three or four of the names can’t be argued with. After that, it gets more tricky. Cases can be made for Bohr, Schrödinger, Pauli, Heisenberg, Curie, Wigner… I could mention a dozen more.

The Bs meson consists of a strange quark and a bottom antiquark, and once it is produced it quickly decays. Very, very rarely it decays into a muon and an antimuon.

The Standard Model of particle physics predicts the rate of the decay of a Bs meson into muons: for every billion Bs mesons that are produced, about 3 of them will decay into a muon-antimuon pair. (The actual figure is 3.54 plus or minus 0.3.)

LHCb, the Large Hadron Collider beauty experiment, has been studying the decay of Bs mesons. (The beauty quark is another name for the bottom quark. Either way, we’re talking here about b quarks.) The experiment says that, for every billion Bs mesons that are produced, about 3 of them decay into a muon-antimuon pair. (The actual figure is 3.2 plus or minus 1.5.) You can find the paper from this CERN webpage. The team hasn’t claimed this as a discovery: the result is at a 3.5 sigma level, which means that there is about a 1-in-4300 chance that the LHCb would see the same bump in their data just due to random chance. But the result is certainly intriguing.

In May 2012 the LHCb collaboration saw this decay of a Bs meson into a muon-antimuon pair (Credit: CERN, LHCb)

Why should this matter? Isn’t it just another case of the Standard Model being proved right? After all, with the discovery of a Higgs (and we’ll soon know for sure whether it’s the Higgs) the Standard Model is on firmer ground than ever. Well, that’s the whole point! The measurement does agree with the Standard Model. But the decay of the Bs into a muon-antimuon pair is believed to be sensitive to physics beyond the Standard Model. In particular there are several models of supersymmetry which, if they were realised in nature, would have the effect of increasing the rate of Bs decay into muons: in these models LHCb should see more than 3 muon-antimuon pairs per billion Bs decays. If the LHCb result stands, then several models of supersymmetry would appear to be ruled out.

Several recent articles have reported the LHCb finding as a significant blow to the whole idea of supersymmetry. Those articles are, I believe, wrong.

First, as the LHCb collects more data it’s possible that deviations from the Standard Model prediction will become evident. Let’s wait and see.

Second, there are other models of supersymmetry that aren’t affected by this result. What’s happening here is that the LHC is narrowing the range in which supersymmetry can be found, just as it narrowed the range where a Higgs could be found – and then found it.

Third, if the LHCb data confirms the Standard Model then the result poses challenges for all ideas for physics beyond the Standard Model. It’s not just supersymmetry that physicists are investigating here, after all.

The result, if confirmed, does raise one disturbing prospect. Perhaps the LHC may not see physics beyond the Standard Model, even when it starts to run at its highest energies. Supersymmetry could still be a phenomenon that applies at really high energies, but we wouldn’t be able to test it with machines such as the LHC. How frustrating would that be?

Most newspaper reports of the recent discovery of a new fundamental boson (let’s agree to call it the Higgs, shall we?) mentioned the long time delay between Higgs postulating the particle and physicists detecting it. That got me to wondering, this morning, whether the time delay was particularly long.

Peter Higgs was the first to postulate the existence of a fundamental scalar particle that might be detectable. He did this in 1964. (Several physicists, at around the same time, argued that a fundamental scalar field was required to give other particles mass; you can read all about that elsewhere.) The point is, it took experimental physicists until 2012 – that’s 48 years – to find the particle and prove that it existed. Is 48 years a long time in this context?

Well, Wolfgang Pauli postulated the existence of the electron neutrino in 1930; it took until 1956 before it was discovered – a lag of 26 years between theory and experiment. There was a similar gap of about a quarter of a century between theorists postulating the existence of the top neutrino and experimenters finding it. (The muon neutrino was discovered a mere 14 years after it was postulated.) So it seems that neutrinos, which are notoriously difficult to study, were found much more quickly than the Higgs.

What about quarks? Well, the bottom and top quarks were postulated in 1973 by Kobayashi and Maskawa; the b quark was found in 1977 (a mere four year later) and the t quark was found in 1995 (a gap of 22 years). So the quark sector was cleared up fairly quickly too.

The W and Z bosons turned up in experiments in 1983, 15 years after Glashow, Salam and Weinberg told people to expect them.

So it would seem that the Higgs is indeed something of a standout amongst the elementary particles: it took almost twice as long to find the Higgs as it did to find any of the other fundamental particles that theorists posited. Personally I’m hoping that the LHC will turn up evidence for a supersymmetric particle. Although supersymmetry itself has a long history, going back to the 1970s, the first realistic supersymmetric version of the Standard Model didn’t arrive until 1981, with work by Georgi and Dimopoulos. If we take 1981 as the starting date, then, it won’t be until 2029 that the Higgs record for a delay between postulation and experiment is broken.

Today’s announcement at CERN, that the CMS and ATLAS experiments have found a boson consistent with the Standard Model Higgs, is the most exciting find in particle physics since … well, since I can remember. The discovery of charm was before my time, but I was a physics student when news of the W and Z discoveries was made public and I don’t believe those announcements matched today’s press conference for drama and sheer emotion (Peter Higgs had to wipe away a tear).

This is a tremendous day for science. Just think what’s happened here. Over a period of decades, theorists and experimentalists developed a theory of the basic interactions (electromagnetism, weak force, strong force) that govern the behaviour of the fundamental particles (quarks, neutrinos, electron, muon and tau). But in order for the theory to match the observed fact that the fundamental particles have mass, theorists had to add something else into the mix. They used purely mathematical reasoning to deduce something incredible about the Universe: that it’s filled everywhere with a scalar field — the so-called Higgs field. It’s the interaction with this field that gives the fundamental particles mass.

And decades after theorists postulated the existence of this field, CMS and ATLAS have found evidence for the associated boson. They saw hints of the Higgs boson last year. Now it’s definite. It has a mass of around 125 GeV.

This is tremendous news for CMS, ATLAS, CERN and science in general. And it’s the start of a whole new era in physics. Now we know where the Higgs is, the LHC — such a tremendous machine — will be able to investigate its properties in detail. And perhaps for the first time we’ll get a glimpse beyond the Standard Model.

Lattice QCD – an approach to solving the theory of quarks and gluons – has been around for ages. I almost took my PhD in it, and that was … mumble, mumble … years ago. Lattice QCD involves formulating QCD in a discrete rather than continuous space-time: formulating it on a grid, or lattice, in other words. You can then use a supercomputer to investigate problems, such as quark-gluon plasma formation or confinement, that are beyond the realm of perturbative field theory techniques.

I had the feeling that practitioners were always promising great things of lattice QCD “next year” (a bit like some physicists have always been saying that cheap, reliable fusion power is “just five years away”). Things are really moving now, though: the computing power that was available back when I was a student is laughable compared to what’s available now. A recent paper in Phys. Rev. Letters demonstrates this: a team of US and UK physicists, running a calculation that took 54 million processor hours on the IBM BlueGene/P supercomputer at the Argonne National Laboratory, have simulated the decay of a kaon into two pions. The results of the calculation agreed with experimental results – but also provided a parameter that was hitherto unknown.

This is really exciting stuff. The discovery of CP violation (the phenomenon that generates the asymmetry we observe between matter and antimatter in the universe) came about through the study of kaon decays. It may well be that lattice QCD will be the tool by which we start to fully understand CP violation.

In December 2012 the ATLAS and CMS teams at the Large Hadron Collider announced that they had seen signals that were consistent with there being a Higgs boson with a mass somwhere in the region of about 124-126 GeV. Statistically, though, they were unable to claim a discovery.

Before Fermilab’s Tevatron collider ceased operations in September 2011 its two experiments – CDF and DZero – generated vast amounts of data that have only now been analysed. On 7 March 2012, scientists announced the results of that analysis at the Rencontres de Moriond conference. The data hint at a Higgs boson with a mass somewhere in the range 115-135 GeV. Again, the statistics fall far short of that required to claim a discovery.

The Tevatron collider at Fermilab, as seen from the air. The main ring and main injector are clearly visible. The ponds are there to dissipate waste heat from the machine.Credit: Fermilab, Reider Hahn

This is tantalising! The ATLAS and CMS teams both make use of high-energy proton-proton collisions produced by the LHC, but they are quite different experiments focusing different things. The CDF and DZero experiments are different again: the Tevatron produced proton-antiproton collisions. So a variety of signals are pointing to a Higgs with a mass somewhere around 125 GeV. But there’s no certainty that it’s there: further data might cause the signal to vanish like the Cheshire Cat.

One thing is certain: by the end of 2012 we will know whether the Higgs exists and, if it does, what its mass is. The LHC is operating so well that there’s now nowhere left for Higgs to hide.

It turns out that there’s a problem with an atomic clock that they used to get start/stop times for the measurement. (The error here would tend to increase the measured time-of-flight, and thus reduce the measured speed.) There was also a problem with the optical fibre connection between the main clock and the GPS system. (Surprisingly, the error here would tend to increase the measured speed.)

The identification of these two systematic errors means that the OPERA team can no longer claim to have seen superluminal neutrinos. Further experiments later this year, both at OPERA and elsewhere, will surely put the story to bed once and for all.

What has been fascinating here, though, has been the reaction of the scientific community to the claim. I think we all knew that this result was never going to stand. But that doesn’t mean the OPERA team were wrong to publish. Their initial result caught the public imagination, and their identification of systematic errors in the experiment showed the public how science progresses in the real world.

They showed that science is sometimes messy, sometimes confusing. But they also showed that science is transparent, and eventually it gives us knowledge we can rely on. Well done OPERA.

I’ve just spent the afternoon watching CERN’s live webcast of the latest CMS and ATLAS data (thank you, CERN, for inventing the Web!). After today there’s very little room for the Higgs still to hide.

ATLAS essentially rules out the existence of a Higgs boson, unless the Higgs mass is in the region 115 to 131 GeV. (I guess I should say that, for comparison, the proton mass is about 1 GeV (actually 0.938 GeV)). CMS seems to rule out a Higgs that is more massive than 127 GeV.

What is tantalising is that both experiments saw hints of a Higgs at around about 125 GeV. Unfortunately, the signal was not strong enough to claim a discovery: what they saw might have been a statistical fluke.

The only way to decide the matter is to take more data which is, of course, what the two experiments will do. In a few months time we will know one way or the other. Either the bumps that ATLAS and CMS saw will go away, and we can say that the Higgs doesn’t exist. Or the bumps will get larger and clearer, and we can say that the Higgs exists with a mass of around 125 GeV.

Either way, new physics will be required. Either way, it will be the discovery of the century.